1. Field of the Invention
The present invention relates to a document feeder, and more particularly to a document feeder which picks up one document from a stack of documents and feeds the document at a constant speed over a stationary image reading device.
2. Description of Related Art
As disclosed by U.S. Pat. No. 5,461,468, a type of document feeder which reads an image of a document with a raster input scanner which is parked at a position while feeding the document on a reading position over the parked input scanner at a constant speed is well known. This is called a feed-scan reading type. In the document feeder disclosed by U.S. Pat. No. 5,461,468, a nudger roller which comes down for feeding of a document picks up some documents from a stack on a tray, and a first feed roller and a retard roller separate one from the other documents. Then, take-away rollers and registration rollers feed the single document toward the reading position correcting possible skew of the document, and a plurality of CVT (constant velocity transport) rollers transport the document on the reading position at a constant speed. In order to minimize the interdocument gap, the nudger roller, the first feed roller and the take-away rollers are driven at a comparatively high speed, and then, when the leading edge of the document reaches the registration rollers, the speed is lowered. The registration rollers and the CVT rollers are driven at the same speed.
With respect to such a feed-scan reading type document feeder, even a little change of the document transport speed on the reading position deteriorates the accuracy of image reading. Fluctuation of the transport speed occurs unavoidably because of the difference among documents in material and/or size and stain and abrasion of the rollers. Especially when a mechanism for driving the CVT rollers on or near the reading position and the roller which is located upstream of the CVT rollers at the same speed is adopted, the following problems occur. If the transport speed of the upstream roller slows down, a tensile force is applied to the document by the CVT rollers. Also, the load on the CVT rollers and the load on the driving motor change suddenly when the trailing edge of a document passes the upstream roller, and fluctuation of the transport speed on the reading position becomes too large to be allowed.